Plastic sacks that can be used in packaging materials heated to temperatures between 180° F. (82° C.) and 260° F. (126° C.), such as cement, sand, and salt, have gained recent interest. Such intended uses have become known as “hot fill applications”. As of the end of 2006, the global market for plastic sacks in hot fill applications has been thought to exceed 20 million pounds per year of bags.
Heavy-duty sacks for cold fill applications are known. See, for example, U.S. Publication No. 2005/0037219 and U.S. Pat. No. 6,368,545. However, such cold fill applications are designed for products that are packaged at temperatures less than 180° F. (82° C.), typically lawn-and-garden products, consumer goods and chemicals. Conventional sacks for such cold fill applications typically include coextruded films made with a metallocene catalyzed polyethylene resin in the skins, a high density polyethylene (HDPE) resin in the core and a low density polyethylene (LDPE) resin throughout the entire structure. This solution is not suitable for hot fill applications because polyethylene resins melt at much lower temperatures, around 255° F. (124° C.), than those encountered in hot fill applications.
Homopolymer polypropylene (PP) resins can provide hot fill capability because such resins melt at 329° F. (165° C.). However, sacks made using homopolymer polypropylene resins have suffered from production deficits because of poor melt strength and delamination problems.
In addition, the poor melt strength of homopolymer PP film resins restricts using these PP resins to such non-film applications as injection molding applications that do not require melt strength, and prevents using homopolymer PP resins in non-film applications requiring melt strength such as blow molding applications without (1) increasing the PP resin's molecular weight, (2) incorporating ethylene into the resin to make copolymer PP resins, (3) incorporating EP rubbers into the PP resins to make EP-PP blends, or (4) incorporating highly branched resins into the PP resins, such as LDPE resin, to make PP blends.
There is a need, therefore, for monolayer and coextruded films employing blends of polypropylene resins to overcome the aforementioned limitations of conventional films.